The long term objective of this research is to contribute to the understanding of the regulation of hemoglobin synthesis and erythroid differentiation in normal and abnormal hematologic conditions. Under conditions of heme deficiency protein synthesis is inhibited due to activation of the heme regulated translational inhibitor (HRI). The focus of the current research is to study the molecular mechanisms in the regulation of HRI which are critical in maintaining the balanced synthesis of heme and globin. The specific aims of this proposal are: (1) Examine the mechanism of heme regulation of HRI by identifying the number of heme-binding sites, by characterizing the heme-binding environment and by localizing the heme -binding domains. (2) Determine the role of autophosphrylation in the regulation of HRI by identification and mutation of the amino acid residues that undergo phosphorylation, and by examination of the effects of these mutations on the function of HRI.